The present invention relates to a method of aircraft traffic indication, and more particularly, to a method of aircraft traffic acquisition and surveillance utilizing minimum interrogation power thereby reducing RF interference and increasing the surveillance coverage.
The increased demands placed on the aircraft flight deck as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring of aircraft traffic in a vicinity of an aircraft (intruder aircraft) that includes automatic identification of potential threats to the monitoring aircraft. As a result, aircraft can have transponders associated therewith that, in response to appropriate electromagnetic interrogation signals induce responding electromagnetic signals that provide information with respect to the range, altitude, and bearing of the interrogated aircraft. Certain traffic control system transponders, e.g., the Mode S system, include target identification as part of the information imposed on the responding electromagnetic signals. For these Mode S systems, the identification of the path or track of the responding aircraft is relatively simple, involving time dependent positions and altitudes of an identified aircraft. Similarly, extrapolations or extensions of aircraft tracks can be relatively simple. The operation of aircraft collision avoidance apparatus is governed by the Radio Technical Commission for Aeronautics (RTCA) Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment manual document number RTCA/DO185.
When TCAS equipped aircraft operate in high density traffic areas, the rf environment can become crowded in the frequency band in which the TCAS and air traffic control (ATC) interrogators and transponders operate. Presently, most acquisition and tracking interrogations are transmitted at power levels that are much higher than necessary. These unnecessarily high power levels contribute to the crowded airwave condition and interference levels in the atmosphere.
Thus there is a need to provide a method which provides for reliable acquisition and tracking of these intruder aircraft without substantially contributing to the rf interference of the environment. The present invention provides a method which uses the received power level and angle of arrival of squitters (periodic unsolicited identity transmissions), altitude fruit (responses to interrogations from interrogators other than own aircraft), or tracking replies to measure the reply path loss, estimate the transmission path loss, and determine the antenna direction, and power level at which the subsequent interrogation should be transmitted. Because this interrogation technique significantly reduces the average TCAS transmitted power, it reduces the need for use of interference limiting procedures. This concept is also very useful in maintaining a larger TCAS surveillance volume relative to using the current algorithms in high traffic density areas. This interrogation technique further increases the safety of the TCAS system. If the low power interrogations are not successful, the intruder is re-interrogated with a power level that is equal to or greater than that used in previous methods. Thus, TCAS may track threatening intruders using this method that it may have missed using the previous methods but it will not miss any intruders it would have tracked using previous methods.